Post-Cure Polymerization and Depth-of-Cure Behaviors of Dental Bulk-Fill Resin-Based Composites

Polymerization for modern dental resin-based composites (RBCs) not only occurs immediately upon light exposure but also continues for another 24 hours, well beyond after light is terminated. However, many questions still remain about the role of polymerization kinetics in optimizing the physical properties of a new RBC type-the bulk-fill.

The aim is to study the post-cure polymerization kinetics of bulk-fill RBCs and to compare their degree of polymeric conversion (DC) and depth-of-cure (DoC) with an incremental-fill, conventional RBC.

Five representative bulk-fill RBCs [Surefil SDR+Stress Decreasing Resin Flow Plus (SDRFP), Tetric EvoCeram Bulk Fill (TECB), Filtek 1 Bulk Fill (F1B), Venus Bulk Fill (VB), and Sonicfill (SF3)] and one conventional RBC [Filtek Supreme Ultra (FSU)] were investigated. The upper surface per RBC specimen was exposed to a light curing unit (Paradigm, 3M-ESPE, irradiance=1221 ± 5 mW/cm2) for 20 seconds. The DC per RBC brand were measured at the bottom surface (specimen Ø=4 mm, thickness=3 mm and 5 mm) as a function of post-curing times using a Fourier transform infrared attenuated total reflection spectrometer. Real-time data recording for post-cure DC began immediately upon light exposure and continued at steady intervals, up to15 min, then again after 24 hours. The DoC of all six RBC brands (n=6 / group) were measured according to ISO-4049. Data were analyzed with nonlinear regression and analysis of variances (ANOVA)/Tukey (α=0.05).

Mean DC for the six RBCs with 5 mm curing height after 24 hours were: TECB=79.5%, VB=75.7%, SDRFP=69.2%, SF3=65.8%, F1B=51.8%, and FSU=44.0%. Bulk-fill RBCs showed higher DC efficiency than the conventional RBC for both the 3 mm and 5 mm curing heights. Significant differences in DoC were found amongst the six RBC brands: VB=5.1 mm, SDRFP=4.6 mm, F1B=3.8 mm, TECB=3.5 mm, FSU=3.0 mm, and SF3=2.7 mm.

DCs were more affected by specimen thickness, through which the curing light was attenuated, than RBC types. Clinician should be aware not all bulk-fill RBCs have a DoC greater than or equal to 4 mm. Also, a bulk-fill RBC that has a high DC after a post-cure time of 24 hours may not have a high DoC, which is typically measured relatively soon after light exposure.